Recently, a water absorbent resin having high absorbency has been developed and is widely used mainly for a disposable purpose, e.g., as an absorbing article such as a disposable diaper and a sanitary napkin, further, as an agriculture/horticulture water retaining agent, an industrial waterproofing agent, and the like. As a material for the water absorbent resin, a large number of monomers and a large amount of hydrophilic polymers have been proposed. Particularly, it is most general that polyacrylic acid and/or a salt thereof water absorbent resin using acrylic acid and/or salt thereof as a monomer is industrially used due to its high water absorbing performance.
As water absorbent properties conventionally desired in the water absorbent resin, there are known a large number of properties (parameters) such as an absorbency without pressure, an absorbency against pressure, a water absorbing rate, a permeability potential without pressure, a permeability potential against pressure, an impact resistance, a anti-urine property, a fluidity, a gel strength, a particle size, and the like, and there have been proposed various definitions (parameter measurement methods) adopted to each property (for example, the absorbency without pressure) from various view points.
These water absorbent resins are used mainly for sanitary materials such as a diaper and a sanitary napkin. Thus, the water absorbent resin is required to be white at the time of shipment from the industry so as not to give a user visually uncomfortable feeling due to coloring in case where powdery water absorbent resin is combined with white pulp in a sanitary material. Further, the water absorbent resin is generally white powder, and it is known that the water absorbent resin is colored (from yellow to brown) with time passage, even after the shipment, during reservation or transport thereof, further, in being used for a sanitary material, so that the water absorbent resin is also required to be white in case where the absorbing article is reserved for an extended period of time.
In order to solve the coloring problem of the water absorbent resin, there were proposed: a method in which acrylic acid monomer and/or salt thereof is polymerized with hydroxyperoxide by a reducing agent and then the polymerized resultant is treated with a silane coupling agent (Patent Document 1); a method in which a water absorbent resin is treated with an organic phosphoric acid compound or salt thereof (Patent Document 2); a method in which a total amount of hydroquinone and benzoquinone in acrylic acid is controlled so as to be 0.2 ppm or less (Patent Document 3); a method in which an inorganic reducing agent is added to a water absorbent resin (Patent Document 4); a method in which organic carboxylic acid or salt thereof is added to a water absorbent resin (Patent Documents 5, 6, and 7); a method in which tocopherol is used as a polymerization inhibitor of acrylic acid so as to carry out polymerization (Patent Document 8); and a method in which a metallic chelating agent is added in producing a water absorbent resin (Patent Documents 9 and 10).
However, each of the foregoing methods results in insufficient improvement in the coloring problem, lower properties, higher cost, and raises a problem in safety depending on a compound to be used.
[Patent Document 1]
    Japanese Unexamined Patent Publication No. 331205/1992 (Tokukaihei 4-331205 (Publication date: Nov. 19, 1992))[Patent Document 2]    Japanese Unexamined Patent Publication No. 86251/1993 (Tokukaihei 5-86251 (Publication date: Apr. 6, 1993))[Patent Document 3]    U.S. Pat. No. 6,444,744 (Publication date: Sep. 3, 2002)[Patent Document 4]    International Publication WO2000/55245 (Publication date: Sep. 21, 2000)[Patent Document 5]    Japanese Unexamined Patent Publication No. 327926/2000 (Tokukai 2000-327926 (Publication date: Nov. 28, 2000))[Patent Document 6]    Japanese Unexamined Patent Publication No. 52742/2003 (Tokukai 2003-52742 (Publication date: Feb. 25, 2003))[Patent Document 7]    Japanese Unexamined Patent Publication No. 186016/2005 (Tokukai 2005-186016 (Publication date: Jul. 14, 2005))[Patent Document 8]    International Publication WO2003/53482 (Publication date: Jul. 3, 2003)[Patent Document 9]    Japanese Unexamined Patent Publication No. 206305/2003 (Tokukai 2003-206305 (Publication date: Jul. 22, 2003))[Patent Document 10]    Japanese Unexamined Patent Publication No. 206381/2003 (Tokukai 2003-206381 (Publication date: Jul. 22, 2003))